I.
Introduction: The bitter rot fungi are almost worldwide in distribution and cause an
especially important disease in the southern areas of the U.S. epiphytotics (outbreaks)
can occur rapidly and losses can be severe, especially under prolonged warm, wet weather.
Several host species can be affected. On peach and nectarine, the same fungi cause a
disease known as anthracnose, and on grape they cause ripe rot. The primary hosts,
however, are apple and pear. The leaf and canker phases of the disease are rare;
therefore, the discussion below is limited to fruit infection.

II. Symptoms: The disease occurs in orchard blocks
beginning in July through August, however, its appearance varies with the climatic
conditions during any particular season. Fruit infection can occur early in the season but
symptoms do not develop until the fruit begins to mature. The rot begins as a small, light
brown, circular lesion. As lesions enlarge, they change to a dark brown and form sunken or
saucer-shaped depressions. The number of lesions per fruit may vary from one to many. When
lesions reach about one inch (25 mm) in diameter, fruiting bodies of the fungus appear
near the center of the lesion. Under humid conditions, large numbers of spores are
produced in a creamy mass, salmon pink in color (photo 2-33), which are often arranged in
concentric circles. Under dry conditions, the spore mass appears crystalline. The rotted
flesh beneath the surface of the lesion is watery, appearing in a V-shaped pattern in
cross section that narrows toward the core (photo 2-34). The fruit decays rapidly as it
ripens and eventually shrivels into a mummy that may remain attached to the tree
throughout the winter. With bitter rot,
the rotten flesh is brown and more watery than would be expected with black rot. White rot lesions appear more cylindrical
when the fruit is cut open. Other decays commonly seen in the orchard at this time of year
are usually initiated at bird pecks or insect injuries. If decays occur on fruit where the
skin has not been damaged, bitter rot is the most likely cause. Unprotected fruit exposed
to high inoculum levels may develop many small dark spots which initially give the fruit a
peppered appearance.

A leaf spot has been associated with Glomerella
cingulata (the perfect stage of Colletotrichum gloeosporioides). Spots begin
as small, red flecks, which enlarge to form irregular brown spots 1/16 to 1/2 inch in
diameter. Severely infected leaves may fall prematurely. Bitter rot cankers are rare in
the eastern United States. When they occur, cankers are target-shaped, i.e. oval, sunken
and often marked with zones or concentric rings.

III. Disease Cycle: The fungus overwinters in
mummified fruit, in cracks and crevices in bark, and in cankers produced by the bitter rot
fungus or by other diseases, such as fire blight. Jagged edges of broken limbs are also
ideal sites. The bitter rot fungus is one of the few rot organisms that can penetrate
unbroken skin of fruit. Although penetration is direct, wounds can be colonized rapidly by
the fungus. Spores are waterborne and are released during rainfall throughout the growing
season. Fruit infection can occur early but is more common from mid to late season. Often,
the first infections appear in cone-shaped areas within the tree beneath mummies or a
canker. Factors which determine the time of appearance of bitter rot are the maturity of
fruit, temperature and humidity, and the presence of disease in the area. The optimum
conditions for disease development include rainfall, relative humidity of 80 to 100
percent, and warm temperatures. Infection can occur in as little as five hours at 79-82 F
(26 C). At 80 F, lesions can develop
and produce spores within 11 days of inoculation. Epidemics can develop rapidly during
prolonged warm, wet weather, and losses can be extensive. The most severe epidemics
usually occur when the early season is warm and wet and primary infection occurs early,
providing abundant secondary inoculum.

IV. Monitoring: Old fire blight cankers,
winter-injured wood, and dead prunings left in the tree often serve as sources of
inoculum. Remove dead wood from the orchard or mulch the brush so that it decays over the
period of a year. Inspect trees for apple mummies and remove them from the orchard if
possible, since mummies remaining in the trees from the previous season can also serve as
a source of inoculum.

During mid-season and continuing through preharvest,
observe 25 fruit on each sample tree. The most disease (photos 2-33, 2-34) is likely to be
found in low areas of the orchard where drying may be slower. 'Nittany' fruits are very
susceptible to bitter rot and may provide an early indication of the disease.

V. Management: Removal of mummified fruit,
dead wood, and twigs killed by fire blight are important sanitation measures that can
reduce the incidence and severity of the disease in some years. Fungicides, applied at
appropriate intervals from petal fall through harvest, are necessary for managing the
disease on susceptible cultivars (Table of effectiveness of
apple fungicides). Removing newly infected fruit from trees during the growing season
may also help reduce the rate of disease spread. Apple
cultivars do not vary widely in their susceptibility to the bitter rot fungi; however,
the disease is often more severe on Empire, Freedom, Golden Delicious, Fuji, Granny Smith,
Nittany and Arkansas Black. The use of a calcium as a nutritional supplement may reduce
the incidence and severity of bitter rot in some years.